A-LevelPhysicsD.C. circuits9702/42 · May/June 2018 · Q78 Marks

Negative feedback is often used in amplifiers. State what is meant by negative feedback,

(a)(i)Negative feedback is often used in amplifiers. State what is meant by negative feedback,[2]
(a)(ii)two effects of negative feedback on the gain of an amplifier.[2]
(b)(i)An ideal operational amplifier (op-amp) is incorporated into the circuit shown in Fig. 7.1. Calculate the gain G of the amplifier circuit.[1]
(b)(ii)1.Determine the output potential difference V_OUT for an input potential difference V_IN of +0.60 V,[2]
(b)(ii)2.-2.1 V.[2]
(b)(iii)The gain of the amplifier shown in Fig. 7.1 is constant. State one change that may be made to the circuit of Fig. 7.1 so that the amplifier circuit monitors temperature with the gain decreasing as the temperature rises.[1]

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Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[M1] (fraction of) output is combined with the input
[A1] output (fraction) subtracted/deducted from input
[B2] Any two valid points e.g. ∞ greater bandwidth/gain constant over a larger range of frequencies ∞ smaller gain
[A1] gain = 1 + (6400 / 800) = 9.0
[A1] (+)5.4 V
[A1] –9.0 V
[B1] replace the 6400Ω resistor with a thermistor

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